In the pulp and paper industry hydrocyclones are widely used for cleaning fibre suspensions from undesired particles and pollution, most commonly heavy particles. Thus the fibre suspension is separated into a heavy fraction containing said undesired heavy particles and a light fraction containing fibres.
In the definition of undesired heavy particles, this comprises particles having higher density compared with the accepted fibres, such as sand, grit, metal, coating flakes and high density plastics. But the undesired particles could also be organic particles originating from wood sources, for example various bark particles, shives, chops, resin particles, vessels and thick wall coarse fibres. The latter ones could have equal density as accepted fibres but is separated due to its lower specific surface.
A typical hydrocyclone plant for this purpose has hydrocyclones arranged in cascade feedback stages.
In order to keep the number of feedback stages down it is important to separate with as high selectivity as possible within each hydrocyclone, i.e. minimize the fibre portion separated and discharged through a heavy fraction outlet of each hydrocyclone, without reducing the share of undesired particles. It is also important to reduce the fibre concentration in the heavy fraction outlet in order to avoid clogging of the heavy fraction outlet at the apex and obtain secure operation conditions.
An aim is to minimize the Thickening factor Tf.Tf=Rm/Rv where Rm is Reject share by mass (ratio of fibres) and Rv is Reject share by volume (ratio of the flow) taken out at the heavy fraction outlet.
In order to minimize the Thickening factor of a hydrocyclone, means for creating turbulence may be provided in the separation chamber. Such examples are described in, for example, EP 615469 B1. Such means for creating turbulence may be a step where the radius of the inside wall of the separation chamber suddenly increases, which causes a turbulent flow expanding flocks of fibres and releasing undesired particles from the fibre network often forming close to the wall of the separation chamber. The steps are parallel with the centre axis of the hydrocyclone.
But there is a need of balancing so that the creating of a turbulent flow expanding fibre flocks will not disturb the helical vortex separating the undesired particles so that the separation efficiency of the hydrocyclone will not be diminished.
Another known hydrocyclone having means for creating turbulence is Celleco Cleanpac 130 made and sold by GL&V Sweden AB. It has a helical path in the circumferential wall of the separation chamber, along a portion of the separation chamber, in the same direction as a helical vortex of the liquid stream when in use. The means for creating turbulence is the same as in EP 615469 B1, i.e. the helical path shows a sudden increase in radius of the separation chamber, one per revolution of the helical path and parallel with the centre axis.